According to Dutch Telecom.paper, Microsoft is trying hard to push Sony Ericsson to use it's Windows Powered Smartphone OS:

Microsoft wants to sign Sony Ericsson as a user of its new Smartphone operating system (OS) for mobile phones. Steve Ballmer, chief executive of Microsoft, has also said that Microsoft has no plans to acquire a stake in Sony Ericsson. Ballmer has said that there are other phone makers prepared to launch phones using its software. Microsoft is hoping for a large share of the market for its mobile phone software.